Scala API: The flow DSL allows the formulation of stream transformations based on some
input.

Scala API: The flow DSL allows the formulation of stream transformations based on some
input. The starting point is called Source and can be a collection, an iterator,
a block of code which is evaluated repeatedly or a org.reactivestreams.Publisher.
A flow with an attached input and open output is also a Source.

A flow may also be defined without an attached input or output and that is then
a Flow. The Flow can be connected to the Source later by using Source#via with
the flow as argument, and it remains a Source.

Transformations can be appended to Source and Flow with the operations
defined in FlowOps. Each DSL element produces a new flow that can be further transformed,
building up a description of the complete transformation pipeline.

The termination point of a flow is called Sink and can for example be a Future or
org.reactivestreams.Subscriber. A flow with an attached output and open input
is also a Sink.

If a flow has both an attached input and an attached output it becomes a RunnableGraph.
In order to execute this pipeline the flow must be materialized by calling RunnableGraph#run on it.

You can create your Source, Flow and Sink in any order and then wire them together before
they are materialized by connecting them using Flow#via and Flow#to, or connecting them into a
GraphDSL with fan-in and fan-out elements.

See Reactive Streams for
details on org.reactivestreams.Publisher and org.reactivestreams.Subscriber.

It should be noted that the streams modeled by this library are “hot”,
meaning that they asynchronously flow through a series of processors without
detailed control by the user. In particular it is not predictable how many
elements a given transformation step might buffer before handing elements
downstream, which means that transformation functions may be invoked more
often than for corresponding transformations on strict collections like
List. *An important consequence* is that elements that were produced
into a stream may be discarded by later processors, e.g. when using the
#take operator.

By default every operation is executed within its own akka.actor.Actor
to enable full pipelining of the chained set of computations. This behavior
is determined by the akka.stream.Materializer which is required
by those methods that materialize the Flow into a series of
org.reactivestreams.Processor instances. The returned reactive stream
is fully started and active.